Poly((R)-β-hydroxybutyrate), i.e., R-PHB, a naturally occurring thermoplastic polyester, shares many of the physical and mechanical properties of poly(propylene) but unlike polypropylene is biodegradable. Despite proven properties and ready applications, industrial production of R-PHB as a bulk polymer using biological methods has, to date, proven economically non-viable. Potential alternate synthetic routes to this promising polymer include Baeyer-Villiger oxidation of an isotactic propylene/carbon monoxide copolymer, asymmetric hydrogenation of the unsaturated polyester from ring opening polymerization (ROP) of ketene dimer, and ROP of (R)-β-butyrolactone (R-BBL). The first two routes involve post-polymerization modification of a polymer backbone, a difficult and unreliable task. The third route uses the proven technology of lactone polymerization but presently the starting material R-BBL is not a readily available commodity such as would be required for commercial polymer production. Thus, for commercial production of R-PHB by the third route, there is a need for an efficient, effective, inexpensive process for producing R-BBL.
The approach with the most promise for producing R-BBL is the catalytic synthesis of R-BBL from R-propylene oxide and carbon monoxide. R-Propylene oxide is readily available through Jacobsen's hydrolytic kinetic resolution as described in Tokunaga, M., et al., Science 277, 936-938 (1997).
Drent, E., et al. European Patent Application No. 0577206 is directed to a process for the carbonylation of expoxides by reaction with carbon monoxide at elevated pressure and temperature in the presence of a catalyst system comprising a source of cobalt and a hydroxy substituted pyridine compound. Data is presented in Example 5 of European Patent Application No. 0577206 where reaction pressurized to 60 bar carbon monoxide at 75° C. for 6 hours, is indicated to give 93% conversion of propylene oxide with a selectivity of greater than 90% into β-butyrolactone (BBL). However, Lee, T. L., et al., J. Org. Chem. 65, 518-521 (1999) and inventors herein were unable to produce the results of Drent and rather obtained low (15%) yields of BBL and significant amounts of undesired oligomeric by-products.
Lee, T. L., et al., J. Org. Chem. 64, 518-521 describes the carbonylation of propylene oxide using 900 psi CO at 80° C. in dimethoxyethane using a mixture of [Ph3P═N═PPh3][Co(CO)4] and BF3.EtO as a catalyst to obtain a yield of 77% BBL (some α-methyl-β-propiolactone also was produced) in a 24 hour reaction. This result leaves room for consideration of other catalyst systems.